But I believe it's uneconomical more for licensing reasons than BOM reasons. Cellular baseband modems are an oligopoly, and so they attach all sorts of rev-share fees to use of their chips, beyond just the cost of the chip itself.
Apple actually owns the baseband/modem group they bought from Intel a couple of years ago. The 11 and 11 Pro were shipped with in-house ex-Intel modems. Since then they have moved back to Qualcomm but I have to imagine they're continuing development and will cut Qualcomm out one day. [1]